Balloon expandable stents have for many years been mounted on angioplasty balloons situated at a distal portion of a balloon angioplasty catheter. When the balloon is inflated at the site of an arterial stenosis, the stent is expanded radially outward thus acting as a scaffold to keep the stenosis from closing after the balloon is deflated and removed from the body. To accomplish this objective, prior art balloons utilize a cylindrical section that extends along most of the longitudinal length of the balloon. A stent is then co-axially mounted at the longitudinal center of this cylindrical section. Because there is typically more resistance to the expansion of the stent at its longitudinal center (as compared to the ends of the stent where there is comparatively little plaque) a deployed stent tends to have an undesirable hourglass shape. That is, pressure from excessive plaque at the longitudinal center of the stent causes that region of the stent to have a reduced diameter.
This problem was addressed in the invention by M. Crocker et al that is disclosed in U.S. Pat. No. 5,843,116. The Crocker et al invention has a longitudinally centered, comparatively short, cylindrical segment that has an increased diameter as compared to the diameter of the balloon cylindrical segments situated both proximal and distal to that central cylindrical segment. Thus, a stent deployed on such a balloon would have an increased diameter at the longitudinal center of the stent. However, it would be extremely rare for a stenosis to have a longitudinal distribution of plaque that is exactly cylindrical for about one third of the longitudinal length of the stent and then becomes a cylindrically shaped, uniformly reduced extent of the plaque both proximal and distal to that center segment. More typically, the plaque has its greatest volume at a central region of a stenosis and then rather uniformly decreases in the extent of the plaque as one moves away from that central region in either a proximal or a distal direction. Thus the Crocker et al invention that has discontinuous (i.e. abrupt) changes in balloon compliance along the length of the balloon is not ideally suited for the placement of a stent in a typical arterial stenosis.
Another problem associated with stents is called "balloon overhang". Balloon overhang is the length of an inflated balloon that extends beyond the edge of the radially expanded balloon. For example, if a radially expanded stent is 17 mm long and mounted on a balloon that has a length when inflated of 20 mm, then the balloon overhang at each end of the stent is 1.5 mm. It has been found in actual practice that a large percentage of restenosis after stent implantation occurs in the region just proximal and just distal to the edge of the stent. This phenomenon, which is known as the "edge effect", is even more pronounced with a radioisotope stent such as described in U.S. Pat. No. 5,059,166 by Fischell et al. One explanation for the edge effect is that trauma to the arterial wall caused by the balloon results in late vascular contraction at that region; and there is no stent structure in that region of balloon overhang to act as a scaffold to prevent the late vasculature contraction.
Prior art balloons all have a conical segment placed at each end of the balloon. The proximal conical end segment is joined at its proximal end to the outer shaft of a stent delivery catheter or balloon angioplasty catheter. The distal conical end segment is joined at its distal end to the distal end of the inner shaft of a balloon angioplasty catheter which balloon angioplasty catheter can also be used for delivering a stent. Early designs for conical end segments had half-angles that were about 45 degrees. Although this half-angle provides a desirable sharp corner where the conical segment joins the cylindrical segment of the balloon, this comparatively high 45 degree half-angle makes it somewhat more difficult for the balloon to be advanced through tortuous coronary vasculature. Therefore, more recent balloon designs have a half-angle for the conical segments that is typically less than 20 degrees. Although this decreased half-angle provides for improved tracking through small curved arteries, the exact length of the cylindrical segment of the balloon onto which a stent would be mounted is more difficult to ascertain. This can lead to a greater length of balloon overhang that results in an increased level of trauma and restenosis beyond the edges of the stent. That is, this can lead to the undesirable result of an increased edge effect.